The present invention relates to a method and a device for the transmission of data between stations of a communications network, in particular for motor vehicles.
Various communications networks, such as those proposed in particular for inter-connecting stations or electronic equipments of a motor vehicle, use a bit-by-bit arbitration access method. In particular, it is possible to mention the following communications protocols.
the J 1850 protocol (SAE) PA1 the C.sup.2 D protocol (Chrysler) PA1 the CAN protocol (Bosch) PA1 PWM coding (Pulse Width Modulation) in which a bit is represented by a rectangular pulse corresponding to the value of the bit, preceded by a dominant pulse and followed by a recessive pulse,
This access method depends on the fact that the physical bus of the transmission networks can have two separate states and that in the case of the simultaneous transmission of the two states, one of the states is systematically given priority and is called the dominant state.
The other state is called the recessive state and corresponds to the free bus state.
When several stations wish to access the network at the same time, after being synchronized at the start of the message, they transmit, in parallel, their respective messages and stop transmitting as soon as, having transmitted a recessive bit, they read a dominant bit on the bus indicating that a station having a greater priority than them is maintaining access to the network.
The conventional techniques for carrying out the bit coding are as follows:
Manchester encoding method in which a bit is represented by a rectangular pulse corresponding to the value of the bit, followed by a pulse of inverse level, and,
NRZ encoding method (Non Return to Zero) in which the bit corresponds to the value of a single rectangular pulse.
For the latter encoding it is necessary to provide a synchronization of a station which is receiving in the case of a succession of bits of the same value.
In the case of use of a simple NRZ encoding method, each transmitted byte is preceded by a dominant start level and followed by a recessive stop level.
This NRZ encoding can be completed by the use of a stuffing bit.
In this case, following a series of n bits of identical level, a bit of opposite value, called a stuffing bit, is inserted during transmission in order to create a transition enabling the resychronization of the stations.
This bit is suppressed during reception.
This encoding method is the most efficient in terms of encoding efficiency and it is that used in the CAN protocol (Bosch).
However, this encoding method is prone to errors and an interference in the network can cause a desynchronization of one or more bits. If this technique is not used, it is then necessary to accept a partial reduction in the encoding efficiency in order to gain reliability.
In fact, the NRZ encoding method (start-stop) has an efficiency of 80% insofar as an 8-bit byte is encoded in ten bits. This efficiency is acceptable but the transitions allowing the resynchronization of the stations can be spaced by nine bits. It is possible to improve this by encoding the information in the form of blocks of four bits in five-bit words, for an equivalent efficiency, the messages including more transitions.
For example, the FDDI (Fibre Distributed Data Interface) optical network has used a block encoding method of this type.
However, in order to provide a shortest possible continuous component, the five-bit words are such that they comprise either two "1s" and three "0s", or three "1s" and two "0s".
The coding obtained is well suited to a token or polling access method, but it is extremely difficult to implement for a bit-by-bit arbitration method.
In fact, the code conversion performed does not enable a transmitted bit to be associated with an encoded bit.